Method for monitoring the course of a process using a reactive gas containing one or several hydrocarbons

ABSTRACT

The method applies to a process such as cementation or chemical vapor infiltration or deposition, the process being carried out in an oven and the method comprising setting operating parameters of the oven, introducing a reagent gas into the oven, the reagent gas containing at least one gaseous hydrocarbon, and extracting from the oven an effluent gas containing reaction by-products of the reagent gas. The effluent gas is subjected to washing in oil that absorbs tars contained in the effluent gas, and information about the progress of the process is obtained by measuring the quantity of tar absorbed by the oil.

BACKGROUND OF THE INVENTION

The invention relates to processes making use of a reagent gascontaining one or more gaseous hydrocarbons, in particular processes forcementation of parts, for forming coatings of pyrolytic carbon onsubstrates by chemical vapor deposition (CVD), or for densifying poroussubstrates with a pyrolytic carbon matrix formed by chemical vaporinfiltration (CVI).

One particular, but non-exclusive, field of application of the inventionlies in making composite material parts comprising a fiber reinforcingsubstrate or “preform” densified by a matrix of pyrolytic carbon, and inparticular carbon/carbon (C/C) composite material parts.

Substrates for densification are placed in an oven into which a reagentgas containing one or more precursors is introduced at low pressure. Theprecursor is constituted by one or more gaseous hydrocarbons, typicallymethane, propane, or a mixture thereof. The operating parameters of theoven are adjusted so as to produce the pyrolytic carbon matrix bydecomposing (cracking) the precursor gas in contact with the substrate.An effluent gas containing reaction by-products is extracted from theoven by pumping.

Usually, the operating parameters of the oven, in particular thetemperature of the oven, the pressure inside the oven, the flow rate ofreagent gas through the oven, and the composition of the reagent gas,are constant throughout the densification process. Unfortunately,infiltration conditions change as the process advances because theinitial pores within the substrate become progressively filled in. Theselected parameters thus result from seeking the best compromise betweenthe optimum that is suitable for the beginning of densification and theoptimum that is suitable at the end of densification, but with a risk ofa modification to the microstructure of the deposited matrix materialbecause of the changing pore size of the substrates, i.e. the changinggeometrical characteristics of the pores. Adapting the operatingparameters of the oven to progress in the densification process couldenable the process to be optimized overall, thereby reducing the timeneeded to obtain a desired level of densification and ensuring that thematrix material is formed with the desired microstructure.

Thus, in document WO 96/31447, the Applicant has proposed varying theoperating parameters of the oven so as to optimize the densificationprocess while controlling the microstructure of the matrix material.Nevertheless, that variation is undertaken in application of apre-established model, and does not take account of the real progress ofthe process.

Proposals are also made in document U.S. Pat. No. 5,348,774 to measurevariation in substrate weight on a continuous basis so as to monitorprogress in the densification process. As a function of the measuredvariation, it is possible to act on various parameters, in particularthe power supplied to an induction coil which serves to heat the oven,by coupling with a susceptor defining the side wall of the oven.Monitoring variation in the weight of the substrates also makes itpossible to detect when the densification process has come to an end.However, this requires the oven to be specially arranged to make itpossible to measure substrate weight on a continuous basis, even at thehigh temperatures that exist in the oven. Such arrangements can also bepenalizing on the substrate-loading capacity in the inside workingvolume of the oven.

OBJECT AND SUMMARY OF THE INVENTION

An object of the invention is to provide a method enabling a process ofdensifying substrates with a pyrolytic carbon matrix to be monitoredcontinuously, without requiring any special arrangement inside theinfiltration oven.

More generally, an object of the invention is to propose a method ofcontinuously monitoring a process implemented in an oven using a reagentgas containing gaseous hydrocarbons at relatively high temperature andleading to the presence of tars in an effluent gas extracted from theoven.

This object is achieved by a method in which, in accordance with theinvention, the effluent gas extracted from the oven is subjected towashing with an oil that absorbs tars contained in the effluent gas,with information about the progress of the process being obtained bymeasuring the quantity of tar absorbed by the oil, advantageously usinga magnitude representative of the variation over time in the quantity oftar absorbed by the oil.

To this end, the oil can be caused to circulate around a closed circuitand the increase in the volume or the weight of the oil is measured.Thus, in an implementation of the method, the oil is taken continuouslyfrom a tank in order to be injected into a stream of effluent gas, withthe tar-filled oil being returned to the tank, and with the informationconcerning the progress of the process being obtained by measuringvariation in the level of the oil in the tank. The oil may be injectedinto a stream of effluent gas flowing in a spray column, e.g. a Venturicolumn.

The oil is preferably an aromatic oil capable of absorbing polycyclicaromatic hydrocarbons contained in the effluent gas. The oil may beselected in particular from aromatic mineral oils such as, for example,oils based on xylenes.

The information obtained about the progress of the process can be usedto determine when the process ends.

It is also possible to control the process by responding to theinformation obtained concerning the progress of the process to modify,where appropriate, at least one of the following operating parameters ofthe oven: oven temperature; pressure inside the oven; flow rate ofreagent gas through the oven; and composition of the reagent gas.

The method of the invention can be used in particular for monitoringprocesses of cementation, chemical vapor deposition, or densification bychemical vapor infiltration.

Monitoring a chemical or physico-chemical process by analyzing reactionby-products in an effluent from an oven or a reactor is a well-knowntechnique. In the particular case of chemical vapor infiltration using areagent gas containing gaseous hydrocarbons such as methane and/orpropane, it might be envisaged that the effluent gas extracted from theoven could be analyzed in order to measure the quantity of someparticular reaction by-product that is produced, for example benzenewhich is a good indicator of how the process is progressing.Nevertheless, such measurement is made difficult to perform because theeffluent gas contains very many light and heavy hydrocarbons including arelatively large quantity of tars that are liable to clog up measurementpipework and equipment quickly.

The Applicant has found that merely monitoring the quantity of tarabsorbed by an oil used for washing the effluent gas provides a reliableindication concerning the progress of the process, without requiringrecourse to complex apparatus and while being entirely suitable forintegration in an installation for processing effluent gas. In order toavoid clogging up pipework and in order to satisfy environmentalrequirements, it is highly desirable to treat the effluent gas so as toeliminate the tars. Eliminating tars by washing in oil is a treatmentmethod that is effective. The present invention can take advantage ofthe use of such a treatment method for the purpose of continuouslymonitoring the process in a manner that is simple and inexpensive.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood on reading the followingindicative but non-limiting description made with reference to the solefigure of the accompanying drawing which shows an industrialinstallation for chemical vapor infiltration enabling a method of theinvention to be implemented.

DETAILED DESCRIPTION OF AN IMPLEMENTATION OF THE INVENTION

FIG. 1 shows in highly diagrammatic manner a chemical vapor infiltrationinstallation for densifying porous substrates with a pyrolytic carbonmatrix. The chemical vapor infiltration process may be of theisothermal-isobaric type, i.e. without using a temperature gradient or apressure gradient across the substrate, or it may be of the temperaturegradient type, i.e. with the substrate being heated non-uniformly, or itmay be of the pressure gradient type, i.e. having different pressures onopposite faces of the substrates.

An oven 10 housed in a casing 12 receives porous substrates 14 fordensification, for example fiber preforms for parts that are to be madeout of carbon-matrix composite material. Examples of fiber preforms arepreforms for divergent portions or divergent portion elements of rocketengine nozzles, or preforms for C/C composite brake disks. The oven 10is defined by a susceptor-forming side wall 16, e.g. made of graphite,together with a bottom 18 and a cover 20, likewise made of graphite. Thesusceptor is coupled with an induction coil 22 that surrounds it. Theoven is heated essentially by radiation from the susceptor which isheated by being inductively coupled with the induction coil.

A reagent gas is introduced via a pipe 24 passing through an inlet 19formed in the bottom 18 of the oven. The reagent gas comprises one ormore carbon-precursor components, possibly together with a dopant. Inthe example shown, the reagent gas is made up of two components comingfrom sources 25 a and 25 b connected to the pipe 24 via valves 26 a and26 b, and via a device 27 for measuring flow rate. An effluent gas isextracted from the oven via an outlet 21 formed in the cover 20 and apipe 28 connected to a pump device 60 which causes reagent gas to flowthrough the oven and which maintains the desired low pressure inside theoven.

The gaseous precursors of pyrolytic carbon are constituted in particularby alkanes, alkyls, and alkenes, typically methane, propane, or amixture thereof, producing carbon by decomposition (cracking) in contactwith the substrates to be densified. The term “dopant” is used herein tomean an optional component of the reagent gas that performs a functionof activating the deposition of pyrolytic carbon from the precursor(s)under selected operating conditions. A dopant may also constitute aprecursor. Thus, in a reagent gas comprising a mixture of methane andpropane (both precursors) coming from sources 28 a and 28 b, the propanemay also act as a dopant under certain conditions of temperature andpressure.

The degree of final densification of the substrates and themicrostructure of the pyrolytic carbon are determined in particular bythe operating parameters of the oven, which are as follows:

-   -   oven temperature;    -   pressure inside the oven;    -   flow rate of reagent gas through the oven; and    -   composition of the reagent gas, namely, in particular, the        precursor content in the reagent gas and the dopant content, if        any.

The effluent gas contains by-products of the (cracking) reaction of thepyrolytic carbon precursor(s), together with those precursor(s) thathave not reacted, and hydrogen gas (H₂) coming from the cracking of theprecursor(s). The reaction by-products include unsaturated hydrocarbons,light aromatic hydrocarbons (benzene, monocyclic hydrocarbons), andpolycyclic aromatic hydrocarbons (PAHs) such as, in particular:naphthalene, pyrene, anthracene, and acenaphthylene which condense inthe form of tars.

In accordance with the invention, the progress of the substratedensification process is monitored by subjecting the effluent gas towashing in an oil that absorbs these tars, and by deriving a magnituderepresentative of the quantity of tar that has been absorbed by the oil.

This washing in oil also makes it possible to eliminate the tars whichwould otherwise clog up the outlet pipework from the oven and whichcould end up in the pumping device used, e.g. in the oil of the vacuumpump or in a condensate from a steam ejector.

An embodiment of an oil washing device 30 interposed between the outletfor effluent gas from the oven 10 and the pumping device 60 is shown inthe accompanying figure. Such a device is also described in the Frenchpatent application filed on the same day as the present application andentitled “Procede et installation pour le traitement de gaz effluentcontenant des hydrocarbures” [A method and an installation for treatingeffluent gas containing hydrocarbons].

The oil washing device is preferably placed close to the outlet from theoven 10 so as to avoid tar deposits forming in the pipe connecting theoven outlet to the washing device, where such deposits are encouraged bythe effluent gas cooling down.

The oil washing device 30 comprises a spray column 32 which is connectedto the pipe 26 at its top end. By way of example, the column 32 is aVenturi column 34 formed by a constriction in the flow section for thegas. At its bottom end, the column 32 communicates with a gas inlet 42formed through the top wall of an oil recirculation tank 40, in thevicinity of one end thereof. A gas outlet 44 also passes through the topwall of the tank 40 and communicates via a pipe 42 with the pumpingdevice 60.

An oil outlet is formed through the bottom portion of the tank 40 and isconnected to a pump 50 which extracts oil from the tank 40 in order tofeed nozzles 36, 38 disposed substantially axially in the column 32, andpassing via a heat exchanger 52. Additional nozzles 46 a and 46 b may beplaced in the tank 40, the nozzles 46 a and 46 b being fed with oildownstream from the heat exchanger 52, in parallel with the nozzles 36,38. The heat exchanger 52 has a cooling fluid, e.g. cold water, passingtherethrough for the purpose of cooling the oil coming from the tank 40.The cooling water also passes through a heat exchanger 54, e.g. in theform of plates, connected in series with the heat exchanger 52 andplaced inside the tank 40.

The heat exchanger 54 and also the nozzles 46 a and 46 b are housedinside the tank between the gas inlet 42 and the gas outlet 44, abovethe level of the oil. A droplet remover 48 may be installed at the gasoutlet 44 from the tank 40.

The oil washing device 30 operates as follows. The oil fed to thenozzles 36 and 38 is sprayed into the flow of effluent gas travelingalong the column 32, with such spraying being encouraged by the increasein the speed of the gas due to the presence of the Venturi. One of thenozzles 36 may be provided at the top end of the column 32, upstreamfrom the Venturi 34, while the other nozzle 38 is provided at the throatof the Venturi. It is also possible to use only one nozzle 36 or 38.

The sprayed oil absorbs a large fraction of the tars conveyed by theeffluent gas, in particular polycyclic aromatic hydrocarbons (PAHs) andthey are entrained into the bath of oil contained in the tank 40.

The oil used must present vapor pressure that is low enough to avoidvaporizing at the pressure that exists at the outlet from the oven 10 soas to avoid loading the effluent gas with oil vapor. As an indication,the pressure inside the oven 10 during a conventional process ofdensifying porous substrates with a pyrolytic carbon matrix is generallyless than 10.1 kilopascals (kPa). The oil must also present viscositythat is low enough to enable it to be circulated and form a mist at theoutlet from the nozzles.

That is why it is preferable to use an aromatic mineral oil with vaporpressure of less than 100 Pascals (Pa) at 0° C.

Advantageously, a xylene-based oil is used such as the synthetic oilsold under the name “Jarithem AX 320” by the French supplier ElfAtochem, which is constituted by 85% by weight mono-xylylxylene and 15%by weight di-xylylxylene. This oil has viscosity of 60 centipoises at 0°C. and a vapor pressure of less than 100 Pa at 0° C.

The heat exchangers 52 and 58 are fed with cold water at a temperatureclose to 0° C. in order to cool as much as possible the oil injected bythe nozzles 36 and 38, and also the oil that is injected via the nozzles46 a and 46 b onto the path between the gas inlet and the outlet of thetank 40.

The heat exchanger 54 contributes to encouraging tars that still existin the effluent gas to condense at the outlet from the column 32.

The droplet remover 48, e.g. of the baffle type contributes to “breakingup” a mist that is present at the outlet from the tank 40 so as toseparate out the droplets and cause them to coalesce so that they can becollected in the bath of oil.

The trapping performed by the oil washing device 30 enables a maximumamount of tars such as PAHs to be eliminated. Only the lightest aromatichydrocarbons (benzenes, monocyclic hydrocarbons) can remain in thewashed effluent gas, however they do not present any risk of cloggingthe pipes because of their higher vapor pressure.

By way of example, the pumping device 60 comprises an ejector-condenser64, or a plurality of similar ejector-condensers connected in series(only one being shown in the figure), it being understood that otherpumping devices could also be used, for example rotary pumps.

The ejector-condenser 64 comprises an ejector portion 66 fed with steamby a boiler 80, and a condenser portion 68 situated downstream from theejector. The condenser 68 is an indirect condenser, the gas coming fromthe ejector being brought into contact with pipes conveying a coolingfluid, e.g. cold water.

After passing through the condenser 68, the water is taken to a coolingtower 70 where it can be collected in a tank 72 from which it isreturned to the condenser 68 by a pump 74.

The condensate picked up in an outlet pipe 76 from the condensercontains hydrocarbons such as benzene, toluene, xylene, and also anyresidue of PAHs dissolved in the water coming from condensation of steamfrom the ejector 66. The condensate may be treated by adsorption onactivated carbon.

At the outlet from the condenser, the effluent gas passes through a pump78. It is possible to use a water level pump cooled by a heat exchangerso that the gas extracted from the treatment installation is practicallyat ambient temperature.

The extracted gas contains essentially unsaturated hydrocarbons, plus aresidue of reagent gas and of hydrogen gas H₂ coming from the oven 10.It can be taken to a torch via a pipe 79 and it can be used at least inpart as fuel gas for the boiler 80. Under such circumstances, it ismixed in a buffer balloon 82 with gaseous fuel, e.g. natural gas, forthe purpose of feeding the burners 86 in the boiler 80.

In the example shown, the quantity of tar absorbed by the washing oil isevaluated by measuring the level of the oil in the tank 40. A sensor 58provides a processor circuit 90 with a signal representative of oillevel for the purpose of generating information I that is usable formonitoring the densification process. The sensor 58 may be of any knowntype, for example of the vibrating blade type or of the radar type, inwhich case it measures the distance between the top wall of the tank inwhich the detector is housed, and the surface of the oil.

The information I is, for example, representative of the derivative withrespect to time of the signal representing oil level, so as to representthe way in which the quantity of tars present in the effluent gas variesover time.

A first possibility consists in monitoring the information I so as todetect the end of the densification process. It can be assumed that thedensification process has been completed once the variation in oillevel, i.e. the variation in the quantity of tars present in theeffluent gas, increases above a given threshold, for example increasesby more than 2% over a given observation period. This may be one toseveral hours, since the duration of densification cycles is usuallyvery long.

Another possibility, which can be used in addition to the firstpossibility, consists in controlling the operating properties of theoven as a function of the value of the information I. Thus:

-   -   when the rate of variation in oil level as measured over a given        observation period drops below a threshold S₁, e.g. about 0.1%,        it is possible to act on the densification parameters (e.g.        temperature, reagent gas composition) so as to amplify        densification; and    -   when the variation in oil level increases to above a threshold        S₂, e.g. about 1%, it is possible to act on the densification        parameters so as to slow densification down.

As an indication, for a nominal flow rate of 500 liters per minute(L/min) of reagent gas admitted to the oven (mixture of methane andpropane), the threshold S₁ may correspond to a decrease in tar weightvariation by an amount lying in the range 0.5 grams per hour (g/h) to 2g/h, and the threshold S₂ may correspond to an increase in the variationof tar weight lying in the range 5 g/h to 8 g/h.

At the end of the process, the tank 40 can be emptied at least in partfrom the outlet of the pump 50 by closing the valve 51 installed in thepipe connecting the pump 50 to the heat exchanger 52, and by opening avalve 53 mounted in a pipe connecting the outlet of the pump 50 to awaste oil outlet 56. The waste oil that is recovered can be destroyed bybeing incinerated, and clean oil is added to the tank 40.

Naturally, means other than measuring the increase in the level orvolume in a recirculation tank could be used for monitoring the quantityof tars absorbed by the washing oil, for example means for measuring theincrease in the weight of the oil.

It should also be observed that variation in the level or the weight ofthe oil in the tank 40 can be converted into a magnitude that isrepresentative of progress in the densification process, for exampleinto an equivalent quantity of benzene contained in the effluent gas.

As already mentioned, the method in accordance with the invention can beimplemented to monitor the progress of processes other thandensification by chemical vapor infiltration, providing the processesare performed in an oven that makes use of a reagent gas containinggaseous hydrocarbons, in particular methane and/or propane, at atemperature that is relatively high, and with tars being produced in aneffluent gas. These other processes include in particular chemical vapordeposition to form coatings of pyrolytic carbon on substrates, whichprocesses are usually performed at a temperature of about 1000° C. orhigher, and cementation of parts in an oven which can make use of amixture of methane and propane at a temperature of about 900° C.

1. A method of monitoring the progress of a process implemented in anoven and using a reagent gas containing at least one gaseoushydrocarbon, said process comprising the steps of: setting the operatingparameters of the oven, introducing the reagent gas containing at leastone gaseous hydrocarbon in the oven, extracting from the oven aneffluent gas containing reaction by-products of the reagent gas, washingthe effluent gas is with an oil that absorbs tars contained in theeffluent gas, and obtaining information about the progress of theprocess by measuring the quantity of tar absorbed by the oil.
 2. Amethod according to claim 1, wherein information concerning the progressof the process is obtained from a magnitude representative of thevariation over time in the quantity of tars absorbed by the oil.
 3. Amethod according to claim 1 wherein the oil is caused to circulatearound along a closed circuit and the increase in oil volume ismeasured.
 4. A method according to claim 3, wherein the oil is takencontinuously from a tank so as to be injected into a stream of effluentgas, the oil loaded with tars is returned to the tank, and theinformation about the progress of the process is obtained by measuringvariation in the level of oil in the tank.
 5. A method according toclaim 1 wherein the oil is caused to circulate along a closed circuitand the increase in oil weight is measured.
 6. A method according toclaim 1, wherein the oil is injected into a stream of effluent gasflowing in a spray column.
 7. A method according to claim 6, wherein theeffluent gas flows through a Venturi column.
 8. A method according toclaim 1, wherein the oil is an oil capable of absorbing polycyclicaromatic hydrocarbons contained in the effluent gas.
 9. A methodaccording to claim 8, wherein the oil is selected from aromatic mineraloils.
 10. A method according to claim, wherein the information obtainedabout the progress of the process is used to determine the end of theprocess.
 11. A method according to claim 1, wherein the informationobtained about the progress of the process is used to control theprocess by modifying, where appropriate and as a function of theinformation obtained about the progress of the process, at least one ofthe following parameters concerning oven operation: oven temperature;pressure inside the oven; flow rate of reagent gas through the oven; andthe composition of the reagent gas.
 12. The use of a method according toclaim 1, for monitoring a process of densifying porous substrates with apyrolytic carbon matrix formed by chemical vapor infiltration.
 13. Theuse of a method according to claim 1, for monitoring a process offorming a pyrolytic carbon coating on substrates by chemical vapordeposition.
 14. The use of a method according to claim 1, for monitoringa process of cementing parts.
 15. A method of monitoring the progress ofa process implemented in an oven and using a reagent gas containing atleast one gaseous hydrocarbon, said process comprising the steps of:setting the operating parameters of the oven, introducing the reagentgas containing at least one gaseous hydrocarbon in the oven, extractingfrom the oven an effluent gas containing reaction by-products of thereagent gas, washing the effluent gas with an oil that absorbs tarscontained in the effluent gas, the washing oil being caused to circulatealong a closed circuit by continuously taking the washing oil from arecirculation tank, injecting the oil taken from the recirculation tankinto a stream of effluent gas and returning the oil loaded with tars tothe recirculation tank, and deriving information about the progress ofthe process by measuring the increase in oil volume or weight in therecirculation tank.